A conventional article of athletic footwear includes two primary elements, an upper and a sole structure. The upper may be formed from a plurality of material elements (e.g., textiles, leather, and foam materials) defining a void that securely receives and positions the foot with respect to the sole structure. The sole structure is secured to a lower surface of the upper and is generally positioned to extend between the foot and the ground. In addition to attenuating ground reaction forces, the sole structure may provide traction and control various foot motions, such as pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of ambulatory activities, such as walking and running.
The sole structure of an article of athletic footwear generally exhibits a layered configuration that includes a comfort-enhancing insole, a resilient midsole formed from polymer foam, and a ground-contacting outsole that provides both abrasion-resistance and traction. Suitable polymer foam materials for the midsole include ethylvinylacetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces. Conventional polymer foam materials compress resiliently, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas. Following repeated compressions, the cell structure of the polymer foam may deteriorate, thereby resulting in an decreased compressibility and decreased force attenuation characteristics of the sole structure.
One manner of reducing the mass of a polymer foam midsole and decreasing the effects of deterioration following repeated compressions is disclosed in U.S. Pat. No. 4,183,156 to Rudy, in which cushioning is provided by a fluid-filled chamber formed of an elastomeric material. The chamber includes a plurality of subchambers that are in fluid communication and jointly extend along a length and across a width of the footwear. The chamber may be encapsulated in a polymer foam material, as disclosed in U.S. Pat. No. 4,219,945 to Rudy. The combination of the chamber and the encapsulating polymer foam material functions as a midsole. Accordingly, the upper is attached to the upper surface of the polymer foam material and an outsole is affixed to the lower surface.
Fluid-filled chambers suitable for footwear applications may be manufactured by a two-film technique, in which two separate sheets of elastomeric film are formed to exhibit the overall peripheral shape of the chamber. The sheets are then bonded together along their respective peripheries to form a sealed structure, and the sheets are also bonded together at predetermined interior areas to give the chamber a desired configuration. That is, interior bonds (i.e., bonds spaced inward from the periphery) provide the chamber with a predetermined shape and size upon pressurization. In order to pressurize the chamber, a nozzle or needle connected to a fluid pressure source is inserted into a fill inlet formed in the chamber. Following pressurization of the chamber, the fill inlet is sealed and the nozzle is removed. A similar procedure, referred to as thermoforming, may also be utilized, in which a heated mold forms or otherwise shapes the sheets of elastomeric film during the manufacturing process.
Chambers may also be manufactured by a blow-molding technique, wherein a molten or otherwise softened elastomeric material in the shape of a tube is placed in a mold having the desired overall shape and configuration of the chamber. The mold has an opening at one location through which pressurized air is provided. The pressurized air induces the liquefied elastomeric material to conform to the shape of the inner surfaces of the mold. The elastomeric material then cools, thereby forming a chamber with the desired shape and configuration. As with the two-film technique, a nozzle or needle connected to a fluid pressure source is inserted into a fill inlet formed in the chamber in order to pressurize the chamber. Following pressurization of the chamber, the fill inlet is sealed and the nozzle is removed.